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Direct Numerical Simulation of Particle Settling in Model Estuaries

Direct Numerical Simulation of Particle Settling in Model Estuaries. R. Henniger (1) , L. Kleiser (1) , E. Meiburg (2) (1) Institute of Fluid Dynamics, ETH Zurich (2) Department of Mechanical Engineering, UCSB. TexPoint fonts used in EMF.

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Direct Numerical Simulation of Particle Settling in Model Estuaries

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  1. Direct Numerical Simulationof Particle Settling in Model Estuaries R. Henniger(1), L. Kleiser(1), E. Meiburg(2) (1) Institute of Fluid Dynamics, ETH Zurich (2) Department of Mechanical Engineering, UCSB TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAAAA

  2. Outline • Introduction / motivation • Computational setup • flow configuration • governing equations and physical parameters • simulation code • Results • freshwater / saltwater mixing • particle settling • Conclusions and outlook

  3. Introduction • Estuary mouth • light fresh-water • heavy salt-water • Suspended particles • e.g. sediment or pollutants • transport out to the ocean • particles settle and deposit • Other influences • temperature profile • Coriolis effect, tide, … • Focus of the present study: basic investigation of • freshwater / saltwater mixing • particle transport, particle settling and particle deposition Magdalena River (Colombia)

  4. Freshwater / saltwater mixing • Typically hypopycnal inflow • (Super-)critical? • Convective mixing, enhanced by • turbulence in river • Kelvin-Helmholtz or Holmboe waves salty salt wedge freshwater salty

  5. Particle load • hypopycnal: • hyperpycnal: freshwater + particles salty freshwater + particles salty

  6. Particle transport (1) (2) (3) • Surface plume • (Enhanced) particle settling • flocculation? • turbulence enhanced settling? • Bottom propagating turbidity current

  7. Model estuary configuration salt sponge convective outflow inflow symmetry planes

  8. Governing equations, non-dimensional • Incompressible Navier-Stokes and concentration transport equations (in Boussinesq regime) • Reynolds number: • Schmidt number: • Richardson number: • Particle settling velocity:

  9. Physical parameters  turbulence • “sharpness” of interfaces  sub-/supercritical flow  particle load  particle plume extent turbulence interfaces inertial forces loading extent

  10. Newly developed simulation code (summary) • Incompressible flows + active scalars • Discretization • compact finite differences in space • explicit or semi-implicit time integration • Massively parallel platform • 3D domain decomposition (>95% parallel efficiency) • sustained 16% peak performance on Cray XT • scalability tested to up to 8000 cores and 17 billion grid points • Validation • convergence orders in time and space • convergence properties of iterative solvers • temporal and spatial growth of eigenmodes • channel flow • shear layer flows with passive scalar • transitional and turbulent channel flow (vs. P. Schlatter) • particle-driven gravity current (vs. F. Necker) • parallel scaling properties

  11. Results:freshwater / saltwater mixing

  12. Freshwater current salt sponge internal waves Kelvin-Helmholtz waves csal= 0.75

  13. Group velocity of internal waves (measured with potential energy at y = 0)

  14. Streamlines on water surface

  15. Sub-/supercritical flow • kinetic vs. buoyant forces • measured with bulk Richardson number

  16. Interface stability • shear stress vs. density difference • measured with gradient Richardson number

  17. Results:particle settling

  18. Particle settling • Three different settling velocities us/U = -0.02, -0.015, -0.01 • Qualitative agreement with laboratory experiments? • Maxworthy (JFM, 1999) • Parsons et al. (Sedimentology, 2001) • McCool & Parsons (Cont. Shelf Res., 2004) • Open questions • extent of particle plume? • particle settling modes (transient, steady state)? • effective settling velocity? • deposit profile?

  19. Particle plume us/U = -0.02, cpart= 0.1 x1 x1 x2 t = 300 t = 400 x1 x1 x2 t = 450 t = 600

  20. Convective particle settling us/U = -0.02 x2= 5 x1= 26 t = 300 x3 x1 x2 t = 350 x3 x1 x2 t = 400 x3 x1 x2 t = 600 x3 x1 x2

  21. Particle mass

  22. Effective particle settling velocity

  23. Particle Deposit us/U = -0.020, t = 710

  24. Particle Deposit us/U = -0.015, t = 920

  25. Particle Deposit us/U = -0.010, t = 1040

  26. Conclusions • Definition of simulation setup • parameters • inflow • boundary conditions • sponge zones, etc. • Results: Basic effects compare well with laboratory experiments • freshwater-brine mixing • finger convection • enhanced convective particle settling • Results obtained at moderate Re and Sc, accessible to DNS

  27. Outlook • Further increase of Re and Sc with LES in the future • Implemented LES models: • ADM-RT model (filter model) • (HPF) Smagorinsky • (upwinding) • Validation of LES to be completed • Further option: more complex domains e.g. by • orthogonal curvilinear grids • immersed boundary method • (immersed interface method)

  28. Appendix

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